What Protestants (this Protestant anyway) share in common with Catholic and Orthodox brethren is a veneration for the ecumenical creeds, formulated at a time of relative spiritual vigor in the church, a faith in the divine wisdom imbued to the men at the Council of Carthage as a means by which God would preserve His word, and in those ordinances explicitly commended in the NT and by Christ Himself. What we reject is any notion that our Lord's kingdom is of this world, or that He is to be worshipped on "this mountain or that." His kingdom is not of this world, not built with brick or stone but through His Word and Spirit.
As to the sufficiency of Scripture, the Bible as a whole, written over centuries by a phalanx of authors, testifies to its own sufficiency in its completeness and excellency through its wisdom, prophecy, and fulfillment, which could in no wise have come from the mind of man, but from God. It needs no external support nor the approval of man.
That's just silly. The Bible nowhere testifies to its own sufficiency, nor completeness. It doesn't even list its own contents.